During the manufacturing process of these fabrics, nano-particles of charcoal are added to the bamboo rayon create the dark grey color. Some claim that the charcoal is antibacterial and also helps with stink in cloth diaper. This is still being studied.The only benefit of the charcoal is to make the insert a dark color. Another thing to be aware of is that many charcoal bamboo inserts are just bamboo on the outside with microfiber sandwiched within. The dark color hides any stains.
For more on bamboo as a material, check https://meraqi.co/blogs/oc/tagged/bamboo-fabric